Liquid crystal display technology has reduced the size of displays from full screen sizes to minidisplays of less than 1.3 inches diagonal measurement, to microdisplays that require a magnification system. Microdisplays may be manufactured using semiconductor integrated circuit (IC) dynamic random access memory (DRAM) process technologies, e.g., liquid crystal on silicon (LCOS). The LCOS microdisplays consist of a silicon substrate backplane with a reflective surface, a cover glass and an intervening liquid crystal layer. The LCOS microdisplays are arranged as a matrix of pixels arranged in a plurality of rows and columns, wherein an intersection of a row and a column defines a position of a pixel in the matrix. To incident light, each pixel is a liquid crystal cell above a reflecting mirror. By changing the molecular orientation of the liquid crystal in the layer, characterized by a tilt angle and a twist angle of the liquid crystal director at any point in the layer, the incident light can be made to change its polarization. The silicon backplane is an array of pixels, typically 7 to 20 micrometers (μm) in pitch. Each pixel has a mirrored surface that occupies most of the pixel area. The mirrored surface is also an electrical conductor that forms a pixel capacitor with the liquid crystal display cover glass electrode which is a transparent conductive coating on the inside face (liquid crystal side) of the cover glass. This transparent conductive coating is typically Indium Tin Oxide (ITO). As each pixel capacitor is charged to a certain voltage value, the liquid crystal fluid between the plates of the pixel capacitors changes its molecular orientation which affects the polarization state of the light incident to the pixels (reflections from the pixel mirrors).
The reflective LCOS microdisplays have a high aperture ratio, and therefore can provide greater brightness than transmissive liquid crystal displays. Major applications of these LCOS microdisplays are in home theater applications, e.g., projectors, and front and rear projection televisions (large screen). For these applications, high contrast is very important. High contrast depends upon the liquid crystal optical mode being used in the liquid crystal display. Typically, a Vertically Aligned Nematic (VAN) mode is one of the optical modes that can achieve a very high contrast and many liquid crystal display manufacturers are beginning to use this particular optical mode in their displays.
The pretilt angle is defined as the tilt angle of the liquid crystal director at the boundary surface. In VAN mode liquid crystal displays, the pretilt angle is small, so the orientation of the molecules of the liquid crystal fluid are nearly perpendicular to the substrate surfaces when there is no electric field applied across the display. Therefore, incoming linearly polarized light, perpendicular to the display substrates, sees a small birefringence as it passes through the layer. Hence this normally incident linearly polarized light experiences little phase retardation when going through the liquid crystal fluid, including being reflected back from the bottom reflective substrate of the display. This provides a very dark “OFF” state when using crossed polarizers (e.g., polarizing beam splitter—PBS), thus very high contrast is achieved. Upon application of an electric field across the liquid crystal fluid, the molecules in the bulk of the liquid crystal fluid orient themselves toward a direction defined by alignment layers on the substrate surfaces, thereby increasing the retardation of the layer of the liquid crystal fluid. Therefore, linearly polarized incident light starts to experience a phase retardation when going into the liquid crystal fluid and then being reflected back from the bottom reflective substrate of the display. As a result of this, the polarization state of the out-going light (reflected light) will be elliptical and some light starts to pass through the crossed polarizers. Increasing the electric field increases this effect until the brightest state is achieved.
In a typical VAN mode, the orientations of the molecules of the liquid crystal fluid at the substrate surfaces are defined by the alignment layers on each of the substrate surfaces. This orientation is described by a pretilt angle and a surface azimuthal direction, which is parallel to the projection of the liquid crystal director onto the plane of the substrate. The azimuthal direction of the molecules of the liquid crystal fluid proximate to the top alignment layer is opposite to the azimuthal direction of the molecules of the liquid crystal fluid proximate to the bottom alignment layer, i.e., anti-parallel. The azimuthal directions defined by the alignment layers are at a 45 degree angle with the direction of polarization of the incoming linearly polarized incident light, as shown in FIGS. 1a and 1b. Usually the pretilt angle of the molecules in a VAN mode display needs to be kept small, e.g., less than 4 degrees, to achieve a very dark “OFF” state, hence the high contrast. Although this pretilt angle is large enough to prevent reverse tilt domains in the display, it is not possible to overcome the defects that occur due to fringe fields between neighboring pixels. Fringe fields become very significant when the pixel size becomes small as is typical in LCOS microdisplays. For example, the size of an LCOS microdisplay may measure one inch diagonally and have a pixel size of approximately 12 μm×12 μm. When high resolution is required, e.g., digital cinema applications, pixel size may be further reduced to approximately 9 μm×9 μm or even smaller. In such situations fringe fields are quite pronounced and the liquid crystals do not align along the direction defined by the tilt direction of the alignment layers. Ultimately this will create defects at the pixel boundaries, usually known as disclinations. This is quite apparent when one pixel is in an “ON” state and an adjacent pixel is in an “OFF” state, wherein the fringe fields are very strong.
To overcome the above problem, it is necessary to increase the pretilt angle generated by the alignment layers on the substrate surfaces. Experimentally it has been determined that the pretilt angle has to be at least 8 degrees to overcome the fringe field effects. However, the dark state of a VAN mode liquid crystal display with a pretilt angle of this magnitude has a significant amount of light leakage through the crossed polarizers and the light contrast it can achieve is not that high. Therefore, the inherent property of VAN displays, the very dark “OFF” state, cannot be fully achieved. This is due to the non-zero birefringence seen by the linearly polarized incident light due to the high pretilt angle of the liquid crystal fluid. Heretofore, it has been necessary to use other methods such as attaching external retarders to stop this light leakage. Generally, this is the current method used by all the VAN liquid crystal display manufactures to solve the above problem.